Acoustic transducer



United States Patent O 3,403,234 ACOUSTIC TRANSDUCER Roswell P. Barnes, Jr., Newton, Mass., assignor to Northrop Corporation, Beverly Hills, Calif., a corporation of California Filed Sept. 11, 1964, Ser. No. 395,684 9 Claims. (Cl. 179-111) ABSTRACT OF THE DISCLOSURE My invention relates to transducers, and more particular- `ly to a novel transducer for converting pressure to a mechanical movement or an electrical voltage, or for converting electrical voltage to a corresponding pressure wave.

For use in such applications as the study of explosive Vblast waves and the like, there is a need for an acoustic transducer capable of responding to a wide range Ifor frequencies in the sonic and infrasonic ranges, over a large dynamic range. Conventional microphones incorporating stressed membranes are capable of great senstivity, but are subject to damage or destruction by mechanical shock or by such acoustic shocks as may occur following an explosion or during a thunderstorm. It is the object of my invention to facilitate the conversion of pressure to voltage over a wide range of pressures and frequencies.

Briefly, the transducer of my invention comprises a capacitor having plates joined by a sheet of compressible foam cemented between them, the bond being uniform and free from voids, and the plates being initially unstressed. In response to pressure variations between the interior and the exterior of the foam which are uniform over the surface of the plates, the plates move toward and away from each other while maintaining their parallel relationship. An electrical amplifier arranged to produce an output voltage controlled by the capacitance between the plates may then be connected across the plates and will produce an output voltage faithfully reproducing the applied pressure in a substantially linear manner over a range of yfrequencies which may extend from the very low infrasonic to the high ultrasonic range. Alternatively, the transducer may be excited by a varying voltage applied across the plates to cause the plates to vibrate and produce an acoustic output signal. While the plates are required to be parallel, they are not required to be plane, but may be curved in any desired manner to produce desired directional characteristics.

In accordance with a preferred embodiment of my invention, three plates are employed, the outer plates being connected together to form a single plate which also serves to shield the center plate. The three plates are maintained in parallel relationship by sheets of compressible non-conductive closed-cell foam extending between them and cemented to their confronting surfaces as described below. Regarding the cementing of the elements, it will be understood that While commonly an actual cement, glue or other adhesive would be employed, it is also contemplated that if a suitable closed-cell foam material is employed having inherently adhesive surfaces or surfaces which can be made adhesive by heat or a suita- 3,403,234 Patented Sept. 24, 1968 ICC ble solvent, it may be joined to the conductive plates without an intermediate cementing material.

For relatively high frequency applications, or should it be desired to use the transducer as a voltage-topres sure conversion device, the plates would be made relatively thin and close together and the foam would be selected to have as high a compressibility as could be attained. On the other hand, there are numerous applications 4for a capacitive microphone in which great physical strength is desired, and for this purpose the plate are made relatively thick and separated by a fairly substantial layer of foam.

The manner in which the transducer of my invention can best be constructed, its mode of operation, and various modifications which may be made in its construction Ifor various purposes, will best be understood in the light of the Afollowing detailed description, together with the accompanying drawings, of various illustrative embodiments thereof.

In the drawings:

FIG. 1 is a schematic diagram, with parts shown in cross-section and parts broken away, of a transducer in accordance with my invention connected as a microphone.

FIG. 2 is a cross-sectional elevation of a specific ernbodiment of the transducer of my invention;

FIG. 3 is a schematic plan view, with parts broken away, of the transducer of FIG. 2;

FIG. 4 is a cross-sectional elevation of a second embodiment of the transducer of my invention;

FIG. 5 is a cross-sectional elevation of a third embodiment of the transducer off my invention; Iand FIG. 6 is a diagrammatic sketch of a fourth embodiment of the transducer of my invention.

Referring iirst to FIG. 1, I have shown a transducer generally designated as 1 connected to a preamplifier. The transducer 1 comprises a variable capacitor having an inner conductive plate 2, of metal or the like, and an outer plate formed by two conductive plates 3a and 3b, also of metal or the like, electrically connected together as indicated at 4. The plates are maintained in a parallel array by intermediate sheets S and 6 of nonconductive compressible foam uniformly cemented to the confronting surfaces of the plates. For optimum performance at very low frequencies, the Ifoam should be of the closed-cell type, for reasons to be described below.

The preamplifier may be of `any desired conventional construction, but as shown comprises a vacuum triode T connected as a plate follower with the grid voltage controlled by the capacitance of the transducer 1. At extremely low frequencies and signal levels, internally generated noise must be minimized, and for this purpose I prefer to use a Nuvistor triode such as the 6CW4 as the triode T.

In a particular embodiment of the apparatus shown in FIG. l, the plates 2, 3a and 3b were each of aluminum. 3 feet by 6 feet by 16 mils in thickness. The sheets 5 and 6 were of natural foam rubber, one-eighth inch in thickness, cemented to the plates with a conventional adhesive dissolved in a solvent and applied with a brush. The static capacitance of the transducer so formed was found to be 12,500 micromicrofarads, and the leakage resistance was 15,000y megohms. The inherent useful frequency range of this transducer is from about 0.001 cycle per second to perhaps 17,000 kilocycles per second. The peramplifer consisted of a 6CW4 triode, a 200 megohm bias resistor R1, a `0.1 microfarad capacitor C1, a 200 megohm -grid resistor R2, a 30,000 ohm plate resistor R3 with a 5 microfarad by-pass capacitor C2, a 1,000 ohm cathode resistor R4 with a 1,000 microfarad electrolytic by-pass capacitor C4, and a 0.68 microfarad output coupling capacitor C3. The voltages Bl-land B2+ were 300 volts and 90 volts, respectively, with respect to ground. These circuit components were selected in the conventional manner for optimum performance in the frequency range of one-half cycle per second to zve cycles per second, in which the voltage gain was 30 db. The transducer 1 was found to have a sensitivity of 0.2 microvolt per microbar per volt of Bl{. Thus, with Bl-lat 300 volts, the grid voltage change produced by an applied pressure differential of 0.1 microbar was 6 rnicrovolts, whereas the internal noise generated in the triode T was less than one microvolt.

The transducer 1 may be constructed in any desired size, and is particularly adapted for use where environmental extremes of heat, pressure or mechanical shock would preclude the use of conventional transducers. When constructed from relatively thick metal plates as described above. the transducer is extremely rugged and will withstand acoustic shock produced by thunderstorms, explosions and the like, or such mechanical stresses as might be produced by livestock walking on its surface.

The sensitivity of the transducer 1 is independent of the area of the plates. However, an important advantage of large area transducers, of, for example, ten to twenty square feet or more in the infrasonic range, is that the effects of wind noise are obviated. In such a transducer, local motion of the plates produced by air turbulence is different at different points on the surface, tending toward a random distribution that produces no net output signal as the area is increased.

'Ihe material from which the sheets 5 and 6 are made is selected on the basis of cost, stability under variations in temperature and over long periods of time, compressibility, uniformity, dielectric constant and resistivity. Typi. cal suitable foams have a' compressibility of about twotenths of the compressibility of air. A high dielectric constant is desirable, and a very high resistivity is necessary. `Open cell forms are most compressible and would be preferred at frequencies well into or above the audible range, but are unsuitable if good response at low frequencies is desired. It is thought that air leakage through the pores of open cell foams at low frequencies occurs before the plates can move, thus preventing their response. Closed cell foams are somewhat less compressible, but permit good response at very low frequencies. Suitable closed cell foams are natural rubber, silicone rubber and neoprene. Silicone foam, in thicknesses of from one thirty-second to one-eighth of an inch, is preferred for use in outdoor environments where a rugged unit capable of resisting shock and varying temperatures is desired. It has been found that a transducer made with silicone foam will vary only percent in sensitivity from about 50 F. to about 250 F., and will operate continuously in this range without deterioration.

It is important that the foam selected be homogeneous, and that the bond to the surfaces of the conductive plates be uniform, i.e., free from voids. An air bubble located anywhere at the boundary of the foam and plates will seriously degrade the performance of the transducer. Thus, care must be taken in cementing the foam to the plates so that wrinkles or gaps in the bond will not occur. The manner in which the foam is cemented to the plates is not criticalv so long a's a uniform bond is obtained. I have employed epoxy resin adhesives, transfer papers and tapes with pressure-sensitive adhesives on both sides, and, for neoprene, a solution of neoprene in a suitable solvent, to cement the foam to the plates.

As noted above, wind noise can be eliminated in the transducer of my invention by making the plates sufficiently large. To insure faithful reproduction of acoustic signals, it is also necessary to exclude stray electromagnetic fields, such as the commonly encountered 60 cycle eld. For this purpose, the transducer of my invention may be constructed as shown in FIGS. 2 and 3. As there shown, the outer plate comprising the plates 3a and 3b is made larger than the inner plate 2. The sheets of foam 5 and 6 may be coextensive with the inner plate 2, as shown, or may extend beyond the inner plate. If desired, additional non-conducting material may be used to fill the remaining space between the plates 3a and 3b. The plates 3a and 3b are preferably bonded together, both electrically and mechanically, by an intermediate metal member 3c, of aluminum, copper or the like, bolted or otherwise secured to them. Alternatively, the member 3c may be formed of known and commercially available conductive epoxy resin formed in place between the plates 3a and 3b.

The intermediate member 3c may extend along only one edge, as shown, or may be made to extend along more than one side or to frame the enclosed plate 2 completely, if desired. In whatever manner the plates 3a and 3b are joined, however, the plate 2 must be located sufficiently far from the joint or joints so that the confronting surfaces of the plates 3a and Sib rea'ct under acoustic stress essentially as though they were not constrained by the joint, enabling them to remain substantially parallel to the plate 2 while moving toward or away from it in response to pressure differences between the interior and the exterior of the foam. For example, in the particular embodiment described above, the central plate 2 might be made 33%I inches by 691A inches, centrally disposed with respect to the outer plates, and located one inch from the member 3c. This mode of operation is characteristic of, and essential to, the transducer of my invention in its most eicient embodiments.

As indicated in FIGS. 2 and 3, an inner conductor 7 may be connected to the inner plate 2, insulated in a conventional manner, not shown, and brought out for connection to the preamplifier through a suitable aperture in the member 3c and a conventional coaxial shielding conductor 8 electrically connected to the outer plate.

While the embodiments of the transducer of my invention thus far described have been shown operating as microphones, it will be apparent to those skilled in the art that the transducer is inherently reversible. Thus, for use as a loudspeaker, a source of varying voltage would be connected between the outer and inner plates, producing an acoustic response.

It is not essential that the outer plate of the transducer of my invention be formed of two separate plates joined together. As shown in FIG. 4, the transducer may be formed of an inner plate 2 with an outer plate 3 wrapped around it, and suitably joined at the ends if so desired to perfect the shielding of the inner plate. The space between the inner and outer plate may be filled with yfoam 9, as by wrapping a sheet of foam around the plate 2, or by conventional techniques for forming the foam in situ. The only requirements are that the foam be of uniform thickness over the surfaces of the inner plate 2, that it be uniformly cemented to the surfaces of the inner and outer plates, and that the inner plate be parallel to the confronting surfaces of the outer plate and far enough from any joints or edges in the outer plate so that the inner and outer plates remain parallel during operation.

The transducer of my invention can be made in essentially any configuration to achieve desired response characteristics. For example, a dat transducer in which the plates were circular and of about 30 feet in diameter would greatly attenuate any wind noise and would be essentially non-directional below about 10 cycles per second, becoming more and more directional as the frequency is increased. A flat circular microphone of this type has the same directional properties as a parabolic microphone having the same diameter. A transducer made with plates 1000 feet long by one foot wide would be highly directional. rl`ransducers in these large sizes are preferably made of a number of smaller transducers electrically, but not mechanically, connected in parallel. For omnidirectional response, the plates 2 and 3 and the foam sheet 9 could be ma-de in the form of concentric spheres, as shown in FIG. 5. For greater directionality, to serve as a panel :microphone to cover a large area such as a stage or the like, the plates could be curved as shown in FIG. 6. Also, while the transducer of my invention is ideally suited for outdoor operation as a microphone, for such purposes as `blast wave studies and the like, it is equally well suited for use as a hydrophone.

While I have described my invention with reference to the details of various specific embodiments thereof, many changes and variations will be apparent to those skilled in`the art upon reading my description, and such can obviously be made without departing from the scope of my invention.

Having thus described my invention, what I claim is:

1. A transducer, comprising, in combination, a sheet of nonconductive closed-cell compressible lfoam of uniform thickness, and first and second opposed parallel electrically conductive plates uniformly cemented to opposite sides of said sheet both of said plates being responsive to acoutic disturbances to compress said foam.

2. The transducer of claim 1 in which said first and second opposed parallel electrically conductive plates are in the configuration of concentric spheres and said sheet of foam as in the form of a sphere filling the volume between the concentric spheres formed by said first and sec-ond conductive plates.

3. The transducer of claim .1, further comprising means connected to said plates and responsive to the capacitance between the plates for producing an output voltage in accordance with said capacitance.

4. The transducer of claim 1, further comprising a source of varying voltage connected across said plates to produce an acoustic output signal.

5. The transducer of claim 1, in which said foam is a member of the class consisting of natural rubber, silicone rubber, and neoprene.

6. An acoustic transducer, comprising three opposed parallel conducting plates, an electrical connection between Athe outer plates, and resilient means bonding confronting surfaces of each of the plates for motion toward and away from each other in parallel array in response to an applied acoustic disturbance wherein said central plate is free to take a position substantially midway of said outer plates.

7. A transducer, comprising first and second acoustically responsive opposed parallel conducting plates, a conductive member secured to and extending between said plates along one edge thereof, a third acoustically responsive conducting plate located between said first and second plates and parallel thereto, said third plate being smaller than said first and second plates and being located at a predetermined distance from said conductive member sufficient to permit all of the plates to remain parallel while deflected together or apart, and smooth sheets of compressible nonconducting foam of uniform thickness located between said first and third plates and having surfaces uniformly cemented to confronting surfaces of said plates to join the plates for resilient movement normal to their surfaces in response to an applied pressure gradient between the interior and the exterior of the foam.

8. The transducer of claim 7, in which said foam is a member of the class consisting of natural rubber, silicone rubber, and neoprene.

9. A transducer, comprising a first electrically conductive plate having first and second opposed parallel surfaces, a second electrically conductive plate substantially surrounding said first plate and having third and fourth surfaces confronting and parallel to said first surface and said second surface, respectively, and two sheets of compressible nonconductive foam of uniform thickness, one sheet extending between said first and third surfaces and the other sheet extending between said second and fourth surfaces, said sheets having opposite surfaces uniformly cemented to the confronting surfaces of said plates.

References Cited UNITEDY STATES PATENTS 1,850,855 3/1932 Thomas 179-111 2,796,467 6/ 1957 Kock 179-111 2,934,612 4/1960 Stanton 179--111 3,008,013 11/1961 Williamson et al. 179-111 3,136,867 6/1964 Brettel 179-111 KATHLEEN H. CLAFF Y, Primary Examiner.

R. P. TAYLOR, Assistant Examiner. 

